HDLC & PPP: Frame Formats, Flow Control & Point-to-Point Links

TAKEAWAYS:

• HDLC and PPP are bit-oriented data link protocols that encapsulate network layer packets into frames for reliable transmission over point-to-point links.
• HDLC uses three frame types (I-frame, S-frame, U-frame) with 0x7E flags and bit stuffing to avoid flag emulation, while PPP uses LCP (Link Control Protocol) and NCP (Network Control Protocol) for negotiation.
• Go-Back-N ARQ improves efficiency over Stop-and-Wait by allowing multiple unacknowledged frames (sliding window) but requires sequence numbers and retransmissions of all frames after the first error.
• PPP supports multiple network layer protocols (IPv4/IPv6) via Protocol Field (0x0021 for IPv4) and includes error detection via CRC-16 in its frame trailer.
• Real-world uses: PPP in Ncell’s 4G/5G modems, HDLC in NTC’s SDH/SONET fiber links, and PPP over Ethernet (PPPoE) in Khalti’s internet banking.
• Exam focus: frame format diagrams, ARQ protocol traces, and comparison tables between HDLC/PPP and Stop-and-Wait/Go-Back-N.

1. Why HDLC and PPP? The Role of Data Link Layer Protocols

The Data Link Layer (Layer 2) is responsible for:

• Framing: Breaking network layer packets into frames and adding headers/trailers.
• Error control: Detecting/correcting errors using checksums or ARQ protocols.
• Flow control: Managing sender/receiver speeds to prevent buffer overflows.
• Access control: Regulating when devices transmit (critical in shared media like Ethernet).

HDLC and PPP are point-to-point protocols, meaning they operate between two directly connected devices (e.g., router-to-router, PC-to-modem). Unlike broadcast protocols (e.g., Ethernet), they assume a dedicated link with no collisions.

2. HDLC: The Bit-Oriented Workhorse

2.1 HDLC Frame Format

HDLC (High-Level Data Link Control) is an ISO standard (ISO 3309/4335) designed for synchronous transmission. Its frame has 8-bit fields (unlike byte-oriented protocols like SDLC, which uses 8-bit delimiters).

+------------+------------+------------+---------------+---------------+------------+
| Flag (0x7E)| Address    | Control    | Information   | FCS (CRC-16) | Flag (0x7E)|
+------------+------------+------------+---------------+---------------+------------+

• Flag (0x7E): Marks frame start/end. Contains 01111110 (binary).
• Address: Identifies the secondary station (e.g., 0xFF for broadcast).
• Control: Defines frame type (I-frame, S-frame, U-frame) and sequence numbers.
• Information: Payload (network layer packet).
• FCS (Frame Check Sequence): CRC-16 for error detection.

2.2 HDLC Frame Types

HDLC supports three frame types, each with a unique role:

• Sequence Numbers: I-frames use modulo-8 sequence numbers (SSSS in control field) for ARQ.
• Poll/Final Bits: Used for master-slave communication (e.g., router polling a modem).

2.3 Bit Stuffing in HDLC

HDLC uses bit stuffing to avoid false flags (when 0x7E appears in data):

• After 5 consecutive 1s, a 0 is inserted.
• Receiver removes stuffed 0s after 5 1s.
• Example:
  • Sender data: 1111101111111
  • After stuffing: 111110 0 111110 0 111111 (flags added: 0x7E...0x7E).

Why? Prevents the receiver from misinterpreting 1111110 (6 1s) as a flag (01111110).

3. PPP: The Modern Point-to-Point Protocol

PPP (Point-to-Point Protocol) is the de facto standard for asynchronous/synchronous links (e.g., dial-up, DSL, VPNs). It replaces older protocols like SLIP and HDLC in most modern networks.

sequenceDiagram
    participant Modem as PPP Modem
    participant Router as PPP Router
    Modem->>Router: LCP Configure-Request (Auth: CHAP, Compression: Predictor)
    Router->>Modem: LCP Configure-Ack
    Modem->>Router: IPCP Configure-Request (IP: 192.168.1.1)
    Router->>Modem: IPCP Configure-Ack
    Modem->>Router: IPv4 Packet (Protocol=0x0021)
    Router->>Modem: ACK (FCS=CRC-16)
    note right of Router: Link remains active until LCP Terminate-Request

3.1 PPP Frame Format

PPP uses a simpler 3-field structure (no address/control fields for point-to-point):

+------------+------------+---------------+---------------+------------+
| Flag (0x7E)| Address    | Control       | Protocol      | Payload    | FCS (CRC-16)|
+------------+------------+---------------+---------------+------------+

• Flag: Same as HDLC (0x7E).
• Address: Always 0xFF (broadcast).
• Control: Always 0x03 (unnumbered frame).
• Protocol: Identifies network layer protocol (e.g., 0x0021 = IPv4, 0x0057 = X.25).
• FCS: CRC-16 (same as HDLC).

3.2 PPP’s Two-Phase Operation

PPP establishes a link in two phases:

1. Link Establishment (LCP):
   • Negotiates encapsulation, authentication (PAP/CHAP), and compression.
   • Uses LCP frames (e.g., Configure-Request, Configure-Ack).
2. Network Layer Protocol (NCP):
   • Configures IPv4/IPv6 (via IPCP), IPX, etc.
   • Example: IPCP Configure-Request assigns an IP address.

Example Trace (PPP Dial-Up):

1. Modem connects → PPP Link Established (LCP).
2. LCP negotiates CHAP authentication.
3. IPCP assigns IP (e.g., 192.168.1.100).
4. User data (HTTP/HTTPS) sent via PPP frames.

3.3 PPP vs. HDLC: Key Differences

4. Go-Back-N ARQ: The Sliding Window Protocol

ARQ (Automatic Repeat reQuest) is an error recovery mechanism. Go-Back-N is an optimized version of Stop-and-Wait that allows multiple unacknowledged frames.

4.1 How Go-Back-N Works

1. Sender transmits N frames without waiting for ACKs (sliding window).
2. Receiver ACKs the highest in-order frame (cumulative ACK).
3. If a timeout or NAK occurs, sender retransmits all frames after the lost one.

Example Trace (N=4):

Sender Window: [1, 2, 3, 4] (sent)
Receiver ACKs: 3 (frame 3 received)
Action: Sender retransmits 4 (since 3 was ACKed, but 4 may be lost).

4.2 Go-Back-N vs. Stop-and-Wait

Real-World Example: Ncell’s 4G Retransmissions

• When a user streams YouTube, TCP (transport layer) uses Go-Back-N-like logic.
• If a packet is lost, TCP retransmits all packets after the lost one (unless SACK is used).

5. In the Real World

5.1 PPP in Ncell’s 4G/5G Modems

• How it’s used: When you connect to Ncell’s 4G network, your phone uses PPP over LTE (PPPoLTE) to negotiate an IP address.
• Key idea: PPP’s LCP phase authenticates your SIM (via CHAP) and assigns an IP (via IPCP).
• Example: Your phone sends a PPP Echo-Request to check link health, just like a router pinging a neighbor.

5.2 HDLC in NTC’s Fiber Optic Backbone

• How it’s used: NTC’s SDH/SONET fiber links use HDLC-like framing for synchronous data transfer.
• Key idea: HDLC’s I-frames carry MPLS labels (used in NTC’s core routing).
• Example: If a fiber cut occurs in Kathmandu, HDLC’s Go-Back-N ARQ ensures no packet loss by retransmitting corrupted frames.

5.3 PPP over Ethernet (PPPoE) in Khalti’s Internet Banking

• How it’s used: When you log into Khalti’s website, your ISP (e.g., Worldlink) uses PPPoE to authenticate you over Ethernet.
• Key idea: PPPoE encapsulates PPP frames in Ethernet, allowing dynamic IP assignment.
• Example: Your bank transaction data travels as: Ethernet Frame → PPPoE Session → PPP Frame → IP Packet → HTTPS.

6. Exam Tip: What to Focus On

6.1 High-Weightage Topics

• HDLC Frame Format: Memorize the 3 frame types (I/S/U) and bit stuffing rule.
• PPP Frame Fields: Know the Protocol Field values (0x0021 for IPv4).
• Go-Back-N ARQ: Draw a sequence diagram with sender/receiver windows and retransmissions.
• Comparisons: Be ready to compare HDLC vs. PPP and Go-Back-N vs. Stop-and-Wait.

6.2 Common Pitfalls

• Bit stuffing: Forgetting to insert a 0 after 5 consecutive 1s (not 6).
• PPP LCP/NCP: Confusing Link Control (LCP) with Network Control (NCP).
• ARQ examples: Not showing cumulative ACKs in Go-Back-N traces.

6.3 Model Answer Structure

For questions like "Explain HDLC frame formats":

1. Define HDLC (1 mark).
2. Draw the frame (label all fields) (3 marks).
3. Explain bit stuffing (2 marks).
4. Give a real-world example (e.g., NTC fiber) (2 marks).

In the real world

• NTC’s SDH/SONET fiber backbone: Uses HDLC’s I-frames for reliable transmission of IPv4/IPv6 packets between routers, with bit stuffing to prevent flag emulation in high-speed links (e.g., Kathmandu–Pokhara backbone).
• Ncell’s 4G/5G modems: Deploy PPP’s LCP/NCP phases to negotiate IP addresses (via IPCP) and authenticate users (CHAP) before handing off data to the core network.
• Khalti’s internet banking: Implements PPPoE (PPP over Ethernet) to encapsulate PPP frames in Ethernet, ensuring secure authentication before processing transactions over the NTC fiber network.